Cellulose acetate fibers have been used for many years to produce many products such as textile yarns, for making fabrics, and filter tow that is used in the production of filter rods for use in tobacco smoke filters. Cellulose acetate fibers are generally produced by a dry spinning process such as those disclosed in U.S. Pat. No. 2,829,027 and U.S. Pat. No. 2,838,364. The cellulose acetate fibers are generally dry spun from a cellulose acetate spinning solution containing cellulose acetate and acetone with other optional additives such as titanium dioxide. The dry spinning process of producing the cellulose acetate fibers generally produces fibers having an average denier per filament of about 2 to 8. Fine denier filaments of cellulose acetate are more preferred for the manufacture of soft and smooth specialty fabrics. Additionally, when used in tobacco smoke filters, cellulose acetate fibers at the lower range of average denier per filament have a greater surface area exposed to the smoke passing through the filter and thus filtration efficiency is increased. In light of the desirable results obtained from the fine denier cellulose acetate fibers, attempts have been made to commercially manufacture reduced denier per filament fibers. Previous attempts to reduce the average denier per filament of cellulose acetate fibers included reducing the viscosity of the spinning solution or spinning dope by reducing the solids content as disclosed in U.S. Pat. No. 3,033,698. However, spinning of this low viscosity spinning solution tends to cause the extruded fibers to stick to the metal surface of the spinnerettes and is thus it is difficult to pull these fibers into a yarn. Methods of producing lower average denier per filament fibers by lowering the solids content also present flow rate control problems and increase the amount of acetone that needs to be recovered. Another method of reducing the average denier per filament of cellulose acetate fibers entails the modification of the holes in the spinnerettes in addition to lower solids content as disclosed in U.S. Pat. No. 3,608,041.
Other methods of producing fine denier cellulose acetate fibers that also entail reducing the viscosity of the solution by reducing solids, correct the stickiness problems by adding metal chelates to the spinning solution such as disclosed in U.S. Pat. No. 3,033,698, U.S. Pat. No. 3,038,780, and U.S. Pat. No. 3,068,063. However, acetone recovery is still a problem and the long term toxicity of these metal chelates is not known, thus these products are not acceptable for tobacco smoke filters. Also end users are reluctant to use fibers with unusual additives.
Another method of reducing the average denier per filament of the fiber entails increasing the draw ratio, however, when producing fine denier fibers by simply increasing the draw ratio, breakage of the filaments is unacceptably high.
It would be very desirable to be able to produce ultra fine denier filaments using a spinning solution of normal to high solids content without dramatically changing spinning conditions or the addition of unusual additives.